Degenerative Mitral Valve Disease in Dogs
Written by Shula Berg BVSc CertAVP(GSAS) GPAdvCert(SASTS) MRCVS
Clinically reviewed by Elizabeth McLennan-Green BVM&S CertAVP(SAM) MRCVS
Table of Contents
The heart contains four chambers; two atria at the top, and two ventricles at the bottom. Deoxygenated blood from the body flows into the right atrium, before passing to the right ventricle, which pumps it towards the lungs. Oxygenated blood from the lungs enters the left atrium, before passing to the left ventricle, which pumps it to the body. The heart valves sit between the atria and ventricles (the mitral valve on the left and tricuspid valve on the right) and between the heart chambers and the great vessels (aortic valve on the left and pulmonary valve on the right). The valves make sure blood flows in one direction by opening and shutting as needed.
Degenerative valve disease (also known as myxomatous valve disease or endocardiosis) is the most frequent form of heart disease diagnosed in dogs. Degeneration of the heart valves stops them forming a complete seal, causing the valves to become ‘leaky’. The most commonly affected valve is the mitral valve, which connects the left atrium and left ventricle. In degenerative mitral valve disease (DMVD), some blood leaks back into the left atrium from the left ventricle when the heart pumps, known as mitral regurgitation. This not only reduces the amount of blood reaching the body with each heartbeat, but gradually causes the left atrium and left ventricle to distend, further reducing the heart’s efficiency.
In early or mild cases of DMVD, the heart is still pushing enough oxygenated blood around the body, so there are no visible symptoms. Mitral regurgitation causes turbulent blood flow, however, which can often be heard as a heart murmur when listening to the heart with a stethoscope. Heart murmurs are often found on examination during a routine exam, such as a vaccination appointment.
Over time, the changes in the heart reduce its ability to function normally. Many dogs will cope with this gradual change, and still may not have symptoms beyond an audible murmur on examination. Eventually, the heart deteriorates to a point known as congestive heart failure (CHF). This means the enlargement of the atrium and ventricle are preventing the heart from pushing sufficient blood around the body. This causes fluid to back up into the lungs (known as pulmonary oedema), which affects the breathing. Symptoms include reduced ability to exercise, faster or laboured breathing, and sometimes collapse. In some dogs, the other valves may develop myxomatous changes, further worsening problems.
Although hearing a murmur tells us that there is a problem with the heart, it does not give us a diagnosis. DMVD is significantly more common in certain breeds, such as Cavalier King Charles Spaniels and Dachshunds, so hearing a murmur in the region of the mitral valve in these breeds can be cause for concern.
A definitive diagnosis is only made using an ultrasound scan of the heart, known as echocardiography (commonly called an “echo”). This is advisable in all cases where a murmur is found, but may require referral to a vet that has had additional training in cardiology. An echo allows us to see the chambers of the heart, assess how they are filling and contracting, and even directly observe turbulent blood flow across the mitral valve. Measurements are recorded, allowing comparison with future scans to monitor progression of disease.
Other tests may be recommended, including blood tests, blood pressure, ECG recording (to check the heart’s rhythm) and chest x-rays. These all provide more information about your dog’s general health, and how their heart function is affecting the rest of their body.
When DMVD is diagnosed, patients will fall in one of five categories:
These are predisposed breeds without current evidence of heart disease.
Dogs in stage B1 have some mitral regurgitation, but without secondary changes to the heart.
Dogs in stage B2 have mitral regurgitation, with secondary enlargement of the atrium and ventricle.
Dogs in stage C have stage B2 changes, and are in congestive heart failure.
Dogs in stage D have congestive heart failure that is no longer responding to treatment, known as refractory congestive heart failure.
Echocardiography is recommended in all patients where a murmur is detected. By diagnosing DMVD when patient are in stage B1 or B2, we have the best chance of slowing down disease progression.
Treatment recommendations vary with the stage of DMVD diagnosed. Patients in stage A or stage B1 do not require treatment, though regular monitoring is advised. This usually includes a check-up, with or without a repeat echo, every 6-12 months.
For patients in stage B2, it is recommended to start daily treatment with a drug called Pimobendan. This helps the heart to pump effectively, and may reduce the amount of mitral regurgitation. A very large scale study demonstrated that giving Pimobendan delayed the onset of congestive heart failure by an average of 15 months. There is no benefit giving Pimobendan to dogs in stage B1, so an echo is required to decide if treatment is needed.
If patients enter congestive heart failure, stage C, they will always need treatment. Dogs can develop symptoms rapidly, and may require immediate veterinary attention. Diuretics are given to relieve fluid build-up in the lungs. Depending on how unwell the dog is, this may be given orally at home, or may be administered by injection in the practice. Other medications may also be prescribed. Some dogs will benefit from a short period of hospitalisation for more intensive care than can be provided as an out-patient. An echo may be required for a current assessment of the heart, and chest x-rays may be recommended to assess the lungs. The majority of cases will respond to treatment, but will need to continue taking oral medication for the rest of their life.
One of the earliest signs of congestive heart failure is an increase in respiratory (breathing) rate. Counting the respiratory rate is an easy way to monitor your pet at home, and is most useful for pets in stage B2, C or D. When your pet is calm and relaxed, count how many breaths they take in 15 seconds (in and out is one breath). Multiply this by four to get a respiration rate per minute. Normal dogs take less than 30 breaths per minute. If you repeatedly count over 40, or see a steady increase over several days, contact your vet. The smartphone app “Cardalis” can help with monitoring and recording respiratory rates.
The prognosis for DMVD is variable, depending on the individual. For the majority of dogs, DMVD progresses over time. However, this is not guaranteed to be the case, and every dog will progress differently. Some dogs may have mild changes and live their whole life without it affecting them. Similarly, although congestive heart failure is progressive, some dogs will remain stable on medication for the rest of their life, while others become refractory and have a guarded prognosis.
The best assessment of prognosis is repeated echo scans, to monitor progression of your individual pet’s heart disease. Some blood tests can also be used to monitor the level of heart muscle damage, and give an indication of response to treatment. Based on these, and clinical examination, your vet will be able to give you a more detailed prognosis.
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Last review date: 13th February 2024
Next review date: 13th February 2026